The pyrolysis mechanisms of many organometallic compounds are of great interest because of their potential use as precursors in the production of semiconductor materials, particularly compound semiconductors of elements from groups HI and V of the periodic table. Often there are competing molecular and radical processes contributing to the overall decomposition mechanisms. This study used electron spin resonance detection of radical intermediates coupled with final product analysis to determine the relative importance of these processes. The very low pressure pyrolyses of triethylgallium, triethylindium and triisopropylgallium were shown to occur by radical pathways and to occur predominantly on the surface of the pyrolysis tube. Isopropyl radicals were observed to further decompose on the surface to yield propene, and this reaction was favoured on Ga-rich surfaces and at high temperatures. A similar reaction was not observed for ethyl radicals on any surface. Gas-phase molecular elimination reactions are shown to contribute only at high pyrolysis temperatures. Hexamethyldisilane was pyrolysed at low and very low pressures and decomposed by a radical process. The very low pressure (10-4 Torr) pyrolysis occurred on the surface of the pyrolysis tube and resulted in the formation of the unexpected (CH3)2Si(H)CH2 radical by an unknown mecanism. At 1 Torr, gas-phase reactions predominated and (CH3)3Si was the major radical formed. Pyrolysis of n5-CH3C5H4Mn(CO)3 gave a vinyl-type radical, the precise identity of which could not be determined, rather than the CH3C5H4 radical proposed in gas-phase pyrolysis studies. CH3Mn(CO)5 and CH3COMn(CO)5 were also shown to decompose by a different mechanism to that observed in gas-phase studies. Pyrolysis of n5- C5H5Mn(CO)3 produced little evidence of surface decomposition and produced radicals suggested by gas-phase studies. The difficulty in identifying the products of the pyrolysis of these compounds prevented the drawing of more detailed conclusions.